Self-healing materials aren’t new. We see the biological processes every day in the form of regeneration of our skin cells or healing of cuts. Self-healing manmade materials – primarily in the form of polymers or elastomers – aren’t new, either. But the ability to 3D print self-healing polymers is rather revolutionary. Researchers at the University of Southern California Viterbi School of Engineering have done just that: created 3D-printed rubber materials that can quite literally fix themselves without human intervention.
The 3D printing method used to create these self-healing objects relies on photopolymerization, a technique that uses light (visible or ultraviolet) to initiate a polymerization reaction to form a linear or crosslinked polymer structure. (Source: Viterbi School of Engineering)
The research team’s process is accomplished using a 3D printing method that relies on photopolymerization, a technique that uses light (visible or ultraviolet) to initiate a polymerization reaction to form a linear or crosslinked polymer structure. Assistant Professor Qiming Wang worked with Viterbi students Kunhao Yu, An Xin, and Haixu Du, and University of Connecticut Assistant Professor Ying Li.
“Photopolymerization is a process that photosensitive polymer liquid solidifies when a light is shined on it,” Dr. Wang told Design News. “We use this process in our 3D-printing system ‘projection stereolithography.’”
Getting the Right Ratio Between Thiols and the Oxidizer
Photopolymerization happens in reaction with a chemical group called thiols. When an oxidizer is added, thiols transform into disulfides, which are able to reform when broken, resulting in materials that can heal themselves. The difficulty for the research team was finding the right ratio between thiols and the oxidizer to achieve the most effective result.
“When we gradually increase the oxidant, the self-healing behavior becomes stronger, but the photopolymerization behavior becomes weaker,” said Dr. Wang. “There is competition between these two behaviors. And eventually we found the ratio that can enable both high self-healing and relatively rapid photopolymerization.”
Then, of course, the next step was to make the resulting material, a silicone rubber or elastomer, 3D printable by turning it into “ink.” To achieve this, the team used a 3D-printer called a “projecting stereolithography system,” an in-house developed printer in two versions: one for single materials and another for multi-materials. In about five seconds, the research team was able to print a 17.5-millimeter square and complete whole objects in about 20 minutes. The multimaterial stereolithography system was first developed in 2016 by Dr. Wang in research on negative thermal expansion in engineered structures.
The result, the researchers found, is a pliable 3D printed material that can fully heal at room temperature in about six to eight hours. The application of heat speeds the process, reducing the self-healing time to one to two hours at a temperature of 60 degrees Celsius, or about 140 degrees Fahrenheit. Humidity has no effect on the healing process. The material is capable of self-healing nearly 100 percent, making it suitable even for critical applications such as tires. It’s expected to be particularly attractive for making shoes, tires and soft robotics, and can even find applications in electronics with some changes to the formula – the addition of carbon grease -- to make it conductive, as well as a little more healing time.
Promising Avenues for Fabricating Structures
To demonstrate the potential applications of the 3D-printable self-healing elastomers, the researchers created a self-healing 3D soft actuator that can lift 10 times its own weight. It’s a nacre-like stiff-soft composite that restores to over 90 percent toughness after fracture, and a self-healable force sensor with both dielectric and conductive phases. The dog-bone-shaped samples, four millimeters thick, were additively manufactured, then cut in two pieces with a blade, brought back into contact with a bit of additional force, and put on a hot plate at 60 degrees Celsius. Afterwards, both original and healed samples were put in a tensile testing machine to examine their performance.
The new material is compatible with photopolymerization-based additive manufacturing systems, and researchers say the photoelastomer is expected to open promising avenues for fabricating structures where free-form architectures and efficient self-healing are both desirable. It’s also reasonably cost-effective at about one dollar per gram.
“Take a typical shoe as an example: the weight of the used rubber may be around 100 grams, and the price of a good shoe may be $200,” Dr. Wang told Design News. “The rubber cost may share $100, so it’s $100/100g, or $1 a gram. Therefore, the cost of our material may be within a similar range of the cost of the existing shoe rubber.”
Seeking Greater Stiffness and New Applications
To further the research in the future, the team is now working to to develop different self-healable materials with increasing stiffness for new applications.
“The next step is to develop 3D-printable and self-healable rigid polymers,” Dr. Wang told Design News. “We want to use those materials to 3D print soldier armors or airplane wings. Just imagine when there are damages and fractures in those structure on the battlefield, they can autonomously self-heal and refunction.”
The research paper entitled, “ Additive manufacturing of self-healing elastomers,” was published in NPG Asia Materials volume 11, Article number 7 (2019). The project was funded by the Air Force Office of Scientific Research Young Investigator Program and the National Science Foundation.
Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine), she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.